The present invention relates to a radiation image conversion panel that has a stimulable phosphor layer formed by a vapor-phase deposition method such as vacuum evaporation and which can reproduce a high-quality radiation image without point defects and any other substantial problems. The invention also relates to a process for producing this radiation image conversion panel.
Upon exposure to a radiation (e.g. X-rays, α-rays, β-rays, γ-rays, electron beams, and ultraviolet rays), certain types of phosphors known in the art accumulate part of the energy of the applied radiation and, in response to subsequent application of exciting light such as visible light, they emit photostimulated luminescence in an amount that is associated with the accumulated energy. Called “storage phosphors” or “stimulable phosphors”, those types of phosphors find use in medical and various other fields.
A known example of such use is a radiation image information recording and reproducing system that employs a radiation image conversion panel having a film (or layer) of the stimulable phosphor (which is hereinafter referred to as a phosphor layer). The radiation image conversion panel is hereinafter referred to simply as the conversion panel and is also called the stimulable phosphor panel (sheet). The system has already been commercialized by, for example, Fuji Photo Film Co., Ltd. under the trade name of FCR (Fuji Computed Radiography).
In that system, a subject such as a human body is irradiated with X-rays or the like to record radiation image information about the subject on the conversion panel (more specifically, the phosphor layer). After the radiation image information is thus recorded, the phosphor panel is scanned two-dimensionally with exciting light to emit photostimulated luminescence which, in turn, is read photoelectrically to yield an image signal. Then, an image reproduced on the basis of the image signal is output as the radiation image of the subject, typically to a display device such as CRT or on a recording material such as a photosensitive material.
The conversion panel is typically prepared by the following method: Powder of a stimulable phosphor is dispersed in a solvent containing a binder and other necessary ingredients to make a coating solution, which is applied to a panel-shaped support (substrate) made of glass or a resin, with the applied coating being subsequently dried.
Also known are conversion panels which are prepared by forming a phosphor layer on a substrate through vapor-phase deposition (vapor-phase film deposition techniques) or vacuum deposition (vacuum film deposition techniques) such as vacuum evaporation and sputtering. The phosphor layer formed by such vapor-phase deposition has superior characteristics in that it is formed in vacuo and hence has low impurity levels and that being substantially free of any ingredients other than the stimulable phosphor as exemplified by a binder, the phosphor layer has not only small scatter in performance but also features very highly efficient luminescence.
When forming the phosphor layer by the vapor-phase deposition, various efforts are being made with a view to improving the characteristics of the conversion panel.
Take, for example, a conversion panel disclosed in JP 2003-50298 A; this panel comprises a substrate and a phosphor layer, between which is provided a matrix layer of stimulable phosphor in the absence of its activator, whereby the phosphor layer is not only improved in adhesion but also grown to have an advantageous columnar structure so as to produce a radiation image that is satisfactory in sharpness, graininess and other properties.
In a conversion panel disclosed in JP 2005-69991 A, the phosphor layer is composed of a lower sub-layer having a spherical crystal structure and an upper sub-layer having a columnar crystal structure, to thereby improve the adhesion between the substrate and the phosphor layer; in addition, the light reflecting characteristics of the lower sub-layer having a spherical crystal structure are combined with the improvement that its presence adds to the columnar nature of the upper sub-layer, thereby achieving a significant improvement in sensitivity.